Light emitting diodes (LEDs) have become very popular for use as lighting devices due to their advantages of high efficiency, long life, mechanical compactness and robustness, and low voltage operation, without limitation. Application areas include liquid crystal display (LCD) backlight, general lighting, and signage display. LEDs exhibit similar electrical characteristics to diodes, i.e. LEDs only conduct current when the forward voltage across the device reaches its conduction threshold, denoted VF, and when the forward voltage increases above VF the current flowing through the device increases sharply. As a result a particular drive circuit has to be furnished in order to control the LED current stably.
The existing approach in today's market normally uses a switching type DC to DC converter, typically in a current control mode, to drive the LED lighting device. Because of the limited power capacity of a single LED device, in most applications multiple LED's are connected in series to form a LED string, and multiple such LED strings work together, typically in parallel, to produce the desired light intensity. In multiple LED string applications a DC to DC converter is normally employed to supply a DC voltage sufficient for the LED operation, however because the operating voltage of LEDs have a wide tolerance (+/−5% to +/−10%), an individual control circuit has to be deployed with each LED string to regulate its current. For simplicity, such a current regulator typically employs a linear regulation technique, wherein a power regulation device is connected in series with the LED string and the LED current is controlled by adjusting the voltage drop across the power regulating device. Unfortunately, such an approach consumes excessive power and generates excessive heat because of the power dissipation of the linear regulation devices. In some approaches a switching type DC to DC converter is provided for each LED string. Such an approach yields a high efficiency operation but the associated costs also increase dramatically.
What is needed, and not provided by the prior art, is an LED drive method with high operating efficiency and a low system cost, which provides a balancing function between the various LED strings of a multiple LED string luminaire.